Zigbee is based on IEEE 802.15.4 and cooja supports 15.4 and 6LoWPAN. So, it should be okay with cooja. But I would recommend ns-3 as it is more resourceful. You can find it here :
https://www.nsnam.org/doxygen/group__sixlowpan.html. But you should remmember that, networking layer of zigbee is different from that of 6LoWPAN though both are based on 802.15.4.